Process for the selective desulphurization of olefinic gasolines, comprising a hydrogen purification step

ABSTRACT

The invention concerns a process for selective hydrodesulphurization of an olefinic gasoline, in which makeup hydrogen and/or the overall feed at the inlet to the catalytic reactor has a CO content of less than 50 ppmv of even less, but a CO x (═CO+½CO 2 ) content of more than 120 ppmv. The invention allows diversification as regards the sources of makeup hydrogen and/or can simplify the purification treatment for said hydrogen, and/or can reduce the purge of hydrogen from the hydrodesulphurization unit.

FIELD OF THE INVENTION

The present invention relates to a process for producing low sulphurcontent hydrocarbons. This invention is principally applicable tomixtures of hydrocarbons which contain a fraction of olefins which isgenerally over 5% by weight and usually over 10% by weight, and at least50 ppm by weight of sulphur. The process allows hydrogen with very lowCO contents, but with relatively high CO₂ contents to be used withoutsignificantly affecting the performance of the catalysts employed duringthe hydrodesulphurization step. This allows diversification as regardsthe sources of possible makeup hydrogen and/or can simplify thetreatment of the hydrogen without resorting to eliminating a lot of CO₂.

PRIOR ART

Future specifications regarding vehicle fuels envisage a substantialreduction in the sulphur content of such fuels, in particular gasoline.In Europe, specifications regarding sulphur contents are at 150 ppm byweight and will reduce in years to come to levels below 10 ppm afterpassing through a 50 ppm by weight level. The change in sulphur contentspecifications thus necessitates the development of novel processes fordeep desulphurization of gasoline.

The principal source of sulphur in bases for gasoline is crackedgasoline, and principally the gasoline fraction derived from a processfor catalytically cracking an atmospheric distillation residue or acrude oil vacuum distillate. The fraction of gasoline from catalyticcracking, which represents on average 40% of the gasoline base,contributes by more than 90% to the amount of sulphur in the gasoline.As a result, the production of low sulphur gasoline necessitates a stepfor desulphurizing catalytically cracked gasoline. That desulphurizationis conventionally carried out using one or more steps for bringing thesulphur-containing compounds contained in said gasoline into contactwith a catalyst in the presence of a hydrogen-rich gas in a processtermed hydrodesulphurization.

Further, the octane number of such gasoline is very high because oftheir high olefin content. Preserving the octane number of such gasolinenecessitates limiting reactions in which olefins are transformed intoparaffins. Such hydrogenation reactions are inherent tohydrodesulphurization processes, which causes a loss of octane numberwhich may be as high as 5 to 10 points, principally due to a reductionin the olefin content.

Further, in refineries, the gasoline hydrodesulphurization process isoften installed on the gasoline cut, directly at the outlet fromcracking units such as catalytic cracking units which usually operatecontinuously for several years. The hydrodesulphurization process mustthus be operated in an uninterrupted manner for 3 to 5 years. Theactivity and quality of the catalysts used to transform sulphur into H₂Smust be high in order to be able to operate continuously for severalyears.

In order to be competitive, hydrodesulphurization processes must satisfytwo principal constraints, namely:

-   -   limited olefin hydrogenation;    -   good stability of the catalytic system and continuous operation        for several years.

Hydrodesulphurization processes are based on treating hydrocarbon cutsover a catalyst containing sulphurized non noble metals and supported.on a mineral support in the presence of hydrogen. The metals usedgenerally contain at least one metal from group VIII (for examplecobalt) and possibly a metal from group VIB (for example molybdenum) ofthe periodic table. The most usual catalytic formulations which areencountered are based on Co and Mo or Ni and Mo deposited on alumina. Inthe case of treatment of olefinic gasoline from cracking units, thecatalyst and operating conditions are optimized to limit the degree ofolefin hydrogenation while maximizing the transformation of organicsulphur-containing compounds to H₂S. Such processes have been describedin particular in European patents EP-A-0 1 031 622 and EP-A-0 1 250 401.

Hydrodesulphurization processes may use hydrogen from a number ofsources. The principal source of hydrogen in the refinery is catalyticreforming. The catalytic reforming unit produces hydrogen during thedehydrogenation of naphthenes to aromatics and dehydrocyclization. Thathydrogen is generally 60% to 90% pure, but is substantially free of COand CO₂.

Depending on refinery requirements, hydrogen may also be produced bysteam reforming light hydrocarbons or by partial oxidation of varioushydrocarbons, in particular heavy residues. Steam reforming consists oftransforming a light hydrocarbon feed into synthesis gas (mixture of H₂,CO, CO₂, CH₄, H₂O) by reaction with steam over a nickel based catalyst.The production of hydrogen by partial oxidation consists of treating ahydrocarbon fraction by high temperature oxidation with oxygen toproduce a synthesis gas constituted by CO, CO₂, H₂ and H₂O. In the lasttwo cases the production of hydrogen is accompanied by a production ofoxides of carbon which are generally substantially eliminated either bymethanation or by adsorption. However, the residual amount of oxides ofcarbon (CO and CO₂) may in some cases be more than 0.50 ppmv, or 100ppmv or even more. Other sources of hydrogen are also occasionally used,such as hydrogen derived from catalytically cracked gas, which containssubstantial quantities of CO and CO₂. Finally, CO and CO₂ may besupplied in some cases by the hydrocarbon feed itself in the form ofdissolved gas if the feed has been in contact with traces of those gasesupstream.

Hydrogen from the refinery and hydrogen from the reaction zone of ahydrotreatment step may thus contain varying quantities of CO and CO₂.The most widely used technique when hydrogen containing CO and CO₂ isused or produced is to completely eliminate those impurities, typicallyby pressure swing adsorption, PSA. That technique is expensive, however,and consumes some of the available hydrogen.

One of the aims of the invention is to operate hydrotreatment stepswell, in particular hydrotreatment steps for selective desulphurizationof olefinic cuts (typically gasoline), while using more diversifiedsources of hydrogen, and typically milder purification treatments.

The invention also aims to reduce the consumption of hydrogen byreducing the hydrogen purge flow rate in the hydrotreatment step (purgeof a portion of the recycle gas around the hydrodesulphurizationreactor).

BRIEF DESCRIPTION OF THE INVENTION

During the course of studies carried out by the Applicant, it wasdiscovered that the presence of CO in the hydrogen, even in amounts ofthe order of 100 ppmv (parts per million by volume) or even 50 ppmv oreven 20 ppmv caused a significant reduction in the activity of thehydrodesulphurization catalysts. Further, it was determined that theolefin hydrogenation reaction rate was little affected by the presenceof CO. The presence of CO in the hydrogen thus caused a reduction incatalytic activity and a greater loss of octane number during thehydrodesulphurization step if the catalytic volume was increased tomaintain the degree of desulphurization. The reduction in activity maybe compensated for by an increase in temperature, but in that case, thecatalyst service life is affected. Similar observations were alsoreported in U.S. patent application 2003/0221994. According to thatpatent, it is recommended to use, for the selectivehydrodesulphurization step, hydrogen containing oxides of carbon inamounts such that the sum CO+½CO₂ (hereinafter designated CO_(x)) mustnot exceed 100 ppmv in the mixture of hydrocarbons and hydrogen.

However, the Applicant has surprisingly discovered that while the COcontent is a parameter directly linked to substantial inhibition of thedesulphurization catalyst, the amount of CO₂ may in contrast vary widelywithout making a significant impact. It was also discovered that ahydrogen pre-treatment step consisting of oxidizing CO to CO₂ withoutextracting the CO₂ thus formed can overcome the deleterious effect ofoxides of carbon even for CO₂ contents of above 200 ppmv. Saving thecost of an expensive step for almost complete elimination of CO₂ is avery significant advantage as well as opening the possibility of usingless pure sources of hydrogen. Thus, the present invention proposes aprocess for desulphurizing hydrocarbon cuts compatible with a very lowCO content but with a substantial CO_(x) content. This processpreferably comprises a step for selective oxidation of the CO containedin the hydrogen to CO₂ and a hydrodesulphurization step, the two stepsbeing carried out in succession, typically with no intermediateextraction of the CO₂ formed. This approach has the advantage over priorart processes of using a simple, cheap solution to overcome problemsregarding the inhibition of hydrodesulphurization catalysts by oxides ofcarbon.

The principal processes for substantially eliminating CO are processesfor oxidizing CO to CO₂, which are preferred in the present invention,and processes for methanation of CO (to methane).

The principal processes for oxidizing CO to CO₂ are the steam conversionreaction which can transform CO to CO₂ by reaction with steam carriedout over a nickel based catalyst, for example, or selective oxidation ofCO to CO₂ using oxygen. This second option (the most preferred in thepresent invention) is described in more detail in the presentapplication.

Methods for selectively oxidizing CO to CO₂ using oxygen have beendescribed in the literature. As an example, International patentapplication WO-A-01/0181242 may be cited, which proposes a method forpurifying hydrogen based on oxidizing CO to CO₂ using a material havinga thermal conductivity of more than 30 W/m.K, to improve the selectivityof the reaction. U.S. Pat. No. 5,789,337 describes a method forsynthesizing catalysts containing finely dispersed gold on a supporthaving increased activity. WO-A-00/17097 recommends the use of catalystscontaining ruthenium or platinum or a mixture of said two elementsdeposited on a support based on a alumina.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for the hydrodesulphurization ofhydrocarbon fractions using a source of makeup hydrogen, and generallyrecycled hydrogen, so that at the inlet to the hydrodesulphurizationreactor, the CO content in the overall feed is at most 50 ppmv,preferably at most 20 ppmv and preferably at most 10 ppmv or less, whilethe amount of CO_(x)═CO+½CO₂ is more than 120 ppmv for at least part ofthe time (for example at least 30% or 50% of the time, or preferably100% of the function time). Usually, the amount of COX is below 10000ppmv, and normally below 5000 ppmv. Typically, the CO_(x) content is inthe range 120 to 1000 ppmv, and more generally in the range 120 to 500ppmv. The process of the invention does not exclude a function in whichfor a part of the time the COX content is below 120 ppmv, or 50 ppmv orstill less. This may, for example, occur when “clean” hydrogen sources,substantially containing neither CO nor CO₂, are available in sufficientquantities to supply the various consuming units (which depends on thenature of the crude oil being processed).

More precisely, the invention proposes a process forhydrodesulphurization of a hydrocarbon HC cut comprising at least 5% byweight of olefins, in which said hydrocarbon cut, a stream HYD of makeuphydrogen, and generally a stream REC of recycle hydrogen are mixed toform an overall feed which is supplied to the inlet to at least onereactor comprising a desulphurization catalyst, under operatingconditions that can transform the organic sulphur-containing compoundsof the HC cut into H₂S, in which the source or sources of hydrogenforming the stream HYD is/are selected, and optionally, at least onehydrogen purification treatment is carried out on the HYD stream, RECstream (or a fraction of REC) or a mixture thereof, so that said overallfeed comprises at most 50 ppmv of CO, and in that it comprises at least120 ppmv of COX for at least a substantial fraction of the time.

Preferably, said purity conditions: (at most 50 ppmv of CO [or 20 or 10ppmv] and at least 120 ppmv of COX for at least a substantial fractionof the time) are, in accordance with the invention, also obtained forthe stream HYD of makeup hydrogen. The best quality makeup hydrogen inthe present invention is that in which the CO content is very low (lessthan 10 ppmv, preferably less than 5 ppmv) and in which the ratio CO₂/COis high (for example more than 5 or 10, for example in the range 5 to60).

Further, when using a hydrogen recycle stream REC around thehydrodesulphurization unit, a purge stream WGAS is also used, to preventimpurities from accumulating. The hydrogen loop tends to concentrate theCO and CO₂. The present invention, which results in the acceptance oflarge quantities of CO₂, can thus increase the REC/HYD recycle ratio,which may exceed 4, or may be in the range 6 to 30. This results in areduction in the required hydrogen purge WGAS. Advantageously, the purgerate is controlled so that the CO content in the overall feed is lessthan 50 ppmv, and preferably less than 20 ppmv, but the COX content inthe overall feed is more than 120 ppmv.

The process typically comprises at least one treatment T1 for purifyinghydrogen carried out on HYD, REC or a mixture thereof, said treatment T1carrying out limited CO₂ elimination leading to a content of at least200 ppmv of CO₂ in the overall feed.

Preferably, the process comprises at least one treatment T2 forpurifying hydrogen carried out on HYD, REC or a mixture thereof, saidtreatment T2 carrying out catalytic oxidation of CO by O₂ and/or H₂O toobtain at most 50 ppmv of CO (and preferably at most 20 ppmv) in theoverall feed.

Oxidation may be carried out by steam CO conversion, which is known asshift conversion, and which may be carried out in one or 2 stages.

Preferably, the process comprises a hydrogen purification treatmentcarried out on HYD, and optionally on REC (or a portion of the REC) or amixture thereof, said treatment comprising a treatment T2 carrying outcatalytic oxidation of CO by O₂ (preferential oxidation of CO over thehydrogen present), directly followed and without secondary CO₂elimination by desulphurization of the HC cut in the presence of astream of the purified hydrogen. It is also possible to combine steamconversion, typically at low temperature, and final preferentialoxidation.

In a variation, the process may comprise at least one treatment T3 forhydrogen purification carried out on HYD, REC or a mixture thereof, saidtreatment T3 carrying out catalytic methanation of CO by H₂ to obtain atmost 50 ppmv of CO in the overall feed. In this case, the processusually comprises a hydrogen purification treatment carried out on HYDand optionally on REC or a mixture thereof, said treatment comprising T3producing methanation of CO by H₂, directly followed without secondaryCO₂ elimination by desulphurization of the HC cut in the presence of astream of the purified hydrogen. While methanation also tends toeliminate the CO₂, the methanation conditions may be such and/orassociated with the presence of CO₂ dissolved in the feed, that theoverall feed nevertheless contains substantial quantities of CO₂ (andCOX possibly over 120 ppmv).

Preferably, the process comprises a hydrogen purification treatmentcarried out on HYD, and optionally on REC (or a portion of the REC) or amixture thereof, said treatment comprising a treatment T2 resulting inthe catalytic oxidation of CO by O₂, directly followed and with nosecondary elimination of CO₂ by desulphurization of the HC cut in thepresence of the purified hydrogen stream.

Frequently, the process also comprises a preliminary treatment T1 foreliminating CO₂ carried out on HYD upstream of T2 or T3, to eliminatethe major portion of the CO₂.

As an example of the production of makeup hydrogen, methane may betreated by steam reforming followed by one or two steam CO conversionsteps and a CO₂ elimination step T1, for example by washing with amethyldiethanolamine solution, to obtain hydrogen with a residual CO lowcontent, for example 2000 to 5000 ppmv, and with a low CO₂ content, inthe range 50 to 1000 ppmv. This hydrogen may then be treated by apreferential oxygen oxidation treatment T2 (or by steam conversionfollowed by preferential oxidation), preferably mixing it with anothersource of very pure hydrogen (hydrogen from catalytic reforming,substantially free of CO and CO₂), at a flow rate suitable to obtain afinal makeup hydrogen with a CO content of 10 or less, or even less than5 ppmv, and a CO₂ content in the range 120 to 1000 ppmv.

Complementary technical features concerning the steam conversion,methanation and CO₂ elimination by amine washing treatments (or otherabsorption liquids) may be found in the reference text “Procédés detransformation” [“Transformation processes”], 1998, by P LEPRINCE,Technip, publishers (Paris), pages 476-490.

Description of Preferred Preferential CO Oxidation Treatment UsingOxygen:

Many catalysts based on supported or unsupported noble metals maycatalyze the oxidation of CO to CO₂ in the presence of oxygen. In thepresence of hydrogen, however, a catalyst has to be used which does nottransform too much of the hydrogen into water. The use of a selectivecatalyst for carrying out the preferential oxidation of CO is thus avery important solution to hydrogen purification problems. A very highdegree of selectivity is not necessary, however, in the context of thepresent invention; the presence of a little steam in the hydrogen thathas been purified over the preferential oxidation catalyst does notcompletely negate the use of hydrogen in a selective olefinic gasolinehydrodesulphurization process. The quantity of H₂S contained in thehydrogen must not generally exceed 10 ppmv (ppm by volume), andpreferably 1 ppmv before the preferential oxidation step. The copperstrip test, which is well known to the skilled person, must be negative.Thus, the hydrogen may optionally be purified of hydrogen sulphide usingany method which is well known to the skilled person. Examples which canbe cited are absorption, extraction or amine washing treatments ortreatments for chemical conversion of the H₂S; this list does not in anyway limit the treatments which may be used in the present invention.

The step for preferential oxidation of CO to CO₂ of the presentinvention may, for example, be carried out on a selective catalyst inthe presence of hydrogen. The metals which may be used to carry out thisreaction may be selected from the group formed by the noble metals Pt,Pd, Ru, Rh, Ir, Au or Cu, Cr, V, Mn or Ce. The metals may be used aloneor in association with other metals, or they may form alloys. They maybe used in the bulk metallic form (filaments, foam, sponge, etc) orsupported on porous refractory oxides such as alumina, cerine, anataseor rutile, zirconia, silica, ferric oxide (α-Fe₂O₃) or zinc oxide.Without in any way limiting the scope of the invention, the preferentialCO oxidation step of the present invention may be carried out on acatalyst based on finely divided gold on ferric hydroxide. Such acatalyst may be prepared using the method described in the publicationby Haruta et al, J Catal 1993, 144, p 175 but it may also be preparedusing any other protocol described in the literature.

The catalyst is, for example, prepared by co-precipitation of a solutioncontaining HAuCl₄.3H₂O and Fe(NO₃)₃.9H₂O and a solution containingsodium carbonate. These two solutions are gradually added then stirredvigorously in a precipitation reactor containing distilled water. Thereaction mixture is maintained at 80° C. while the two solutions areadded; throughout the operation, the pH is kept between 8 and 8.5. Afterfiltration, the precipitate is washed with hot water until the washingwater contains no more chlorine (monitored by the silver nitratereaction) then dried at 40° C. in a vacuum oven for 12 h. The powderobtained is then calcined in dry air at 400° C. for 2 h with a flow rateof air of 0.5 l/g catalyst/h. After milling, a powder with a meangranulometry of close to 20 μm and with a surface area of 60 m²/g isobtained. The catalyst contains 3% by weight of Au.

The catalyst may be formed using any of the methods which are known tothe skilled person; non limiting examples which may be cited aredeposition onto a monolith using a wash-coat (coating deposited in theliquid phase), granulation, extrusion, etc.

Description of Hydrodesulphurization Step

The hydrodesulphurization step is carried out on a catalyst whichcomprises at least one element from group VIII and preferably an elementfrom group VIII and an element from group VIB. The element from groupVIII is selected from the group constituted by nickel, cobalt and iron.The element from group VIB, if present, is preferably molybdenum ortungsten. The metals are deposited onto an amorphous solid supportselected from the group constituted by silica, silicon carbide andalumina, formed into beads or extrudates. To selectivelyhydrodesulphurize carbonaceous fractions containing olefins, it ispreferable to use catalysts containing cobalt and molybdenum on asupport based on alumina.

The hydrodesulphurization step may advantageously be carried out in twosteps, a first hydrodesulphurization step which can transform more than50% of the sulphur present in the feed to H₂S, and a finishing stepconstituted, either by a step for hydrogenolysis of the saturatedsulphur-containing compounds over a catalyst containing a metal fromgroup VIII or by a step for hydrodesulphurization over a catalyst havingan activity which is lower than the catalyst for the first step. Thistype of concatenation may improve the selectivity of thehydrodesulphurization step.

The catalyst or catalysts employed during this step are in thesulphurized form. The sulphurization procedure may be carried out insitu or ex situ. In the first case, the catalyst is sulphurized beforeloading into the reactor, while in the second case, the catalyst isloaded into the reactor in the form of metallic oxides, sulphurizationis carried out in the reactor by injecting H₂S or compounds which maydecompose to H₂S such as DMDS and hydrogen. Any conventionalsulphurization method used by the skilled person which can sulphurize atleast 50% and preferably 70% of the metallic oxides deposited on thesupport may be employed.

The reactor pressure is generally in the range 0.5 MPa to 5 MPa, thehydrogen flow rate is such that the ratio of the hydrogen flow rates innormal litres per hour to the flow rate of hydrocarbons in litres perhour is in the range 50 to 800, preferably in the range 60 to 600. Thetemperature is in the range 200° C. to 400° C., preferably in the range230° C. to 350° C. depending on the amount of sulphur in the hydrocarbonfraction to be desulphurized.

EXAMPLES Example 1 Comparative

A pilot unit constituted by a reactor with a capacity of 200 ml wascharged with 100 ml of commercially available HR806S catalyst sold byAXENS. This catalyst is based on cobalt and molybdenum deposited onalumina and was supplied in the pre-sulphurized form and thus did notrequire a subsequent sulphurization step before contact with the feed.The treated feed was gasoline A from a catalytic cracking unit. Thisgasoline was depentanized to treat only the C₆+ fraction byhydrodesulphurization. This feed contained 425 ppm of sulphur with 6 ppmof sulphur in the form of mercaptans and with a bromine index, measuredusing the ASTM D1159-98 method, of 49 g/100 g. The cut points for thisgasoline A were determined by simulated distillation: gasoline A had 5%by weight and 95% by weight cut points of 61° C. and 229° C.respectively.

Gasoline A was mixed with pure hydrogen and injected into the reactor.The pressure was kept at 2.1 MPa, the feed flow rate was 400 ml/h,representing an hourly space velocity (HSV) of 4 h⁻¹, the hydrogen flowrate was 120 litres per hour, representing a flow rate of 300 (normal)litres of hydrogen per litre of feed. Three different temperatures weretested.

The apparent selectivity of the catalyst was calculated for each pointas being the ratio of the apparent first order rate constants betweenthe rate of desulphurization and the olefin hydrogenation rate.

The sulphur contents and the olefins contents, measured by the bromineindex, and the selectivities are shown in Table 1. TABLE 1 Temperature °C. 260 280 300 Sulphur, test Ppm 64 18 9 IBr, test g/100 g 37.9 31.422.4 Selectivity 7.4 7.1 4.9

Example 2 Comparative

In order to measure the influence of CO and CO₂ on catalyst performance,a cylinder of hydrogen containing 100 ppmv of CO and 350 ppmv of CO₂ wasused. This hydrogen was mixed with gasoline at flow rates identical tothose of Example 1. The mixture thus formed had a CO content of 65 ppmvand a CO₂ content of 228 ppmv. The operating conditions were identicalto Example 1. Table 2 shows the results of the test. TABLE 2 Temperature° C. 260 280 300 Sulphur, test ppm 103 25 13 IBr, test g/100 g 38.4 32.323.6 Selectivity 5.8 6.8 4.8

The presence of CO and CO₂ in respective amounts of 65 ppmv and 228 ppmvin the mixture of hydrogen and gasoline A degrades thehydrodesulphurization activity of the catalyst. In contrast, thehydrogenating activity is virtually unaffected, causing a fall inselectivity.

Example 3 In Accordance with the Invention

Example 3 was in accordance with the invention, i.e. the hydrogencontaining CO and CO₂ used in Example 2 was pre-treated to oxidize theCO to CO₂. Oxidation was carried out by mixing hydrogen and oxygen andtreating the mixture over an oxidation catalyst. The hydrogen was mixedwith a stream of pure oxygen, the flow rate of which was adjusted sothat the molar ratio between the oxygen and the CO was 1.1. The reactorwas operated at a temperature close to ambient temperature (50° C.), ata pressure of 2.1 MPa.

The catalyst was prepared, for example, by co-precipitation of asolution containing HAuCL₄.3H₂O and Fe(NO₃)₃.9H₂O and a solutioncontaining sodium carbonate. These two solutions were gradually added toa precipitation reactor containing distilled water then vigorouslystirred. The reaction mixture was maintained at 80° C. throughoutaddition of the two solutions, and during the entire operation the pHwas maintained between 8 and 8.5. After filtration, the precipitate waswashed with hot water until the washing water contained no more chloride(monitored by reaction with silver nitrate) then dried at 40° C. in avacuum oven for 12 h. The powder obtained was calcined in dry air at400° C. for 2 h with an air flow rate of 0.5 l/g of catalyst/hydrogen.After milling, a powder with a mean granulometry of close to 20 μm andwith a specific surface area of about 60 m²/g was obtained. The catalystcontained a quantity of 3% by weight of Au. Using the Au[111] peak, Xray diffraction analysis allowed a gold particle side of 60 Å to bedetermined. The catalyst (100 mg, diluted in a ratio of 1:20 withα-Al₂O₃) was then disposed in a stainless steel reactor with an internaldiameter of 10 mm and inserted into a jacketed tubular oven heated to50° C. The hydrogen flow rate was adjusted to 5×10⁴ Nml/h/g of catalyst,so that the hourly space velocity was 5×10⁵ h⁻¹. The density of thecatalyst was 1 g/cm³. An analysis of the gas entering the reactor andthat of the effluents (CO, —CO₂, H₂O, O₂) was carried out by gaschromatography having two catharometric detectors.

After passing hydrogen over the preferential oxidation catalyst underthe conditions described, the stabilized CO and CO₂ content in thetreated hydrogen was 17 ppmv and 430 ppmv respectively.

A cylinder of pressurized hydrogen gas containing about 17 ppmv of Coand 430 ppmv of CO₂ was produced following these results. The gas itcontained was then mixed with gasoline A and sent to the reactor used inExamples 1 and 2 under the same operating conditions. The mixture thusconstituted had a CO content of 11 ppmv and a CO₂ content of 283 ppmv.Table 3 shows the results of the tests. TABLE 3 Temperature ° C. 260 280300 Sulphur, test ppm 71 20 10 IBr, test g/100 g 38.1 31.8 22.9Selectivity 7.1 7.1 4.9

Carrying out an oxidation step to pre-treat the hydrogen significantlyimproved the hydrodesulphurization activity of the catalyst and producedactivities and selectivities close to those of the tests carried outwith hydrogen free of CO and CO₂ described in Example 1.

As a result, the use of an oxidizing pre-treatment step for CO can, bymeans of a simple device, considerably limit the deleterious effect ofthe presence of CO in hydrogen on the performance ofhydrodesulphurization catalysts, without the need to eliminate CO₂ toany great extent.

1. A process for hydrodesulphurization of a hydrocarbon cut HCcomprising at least 5% by weight of olefins, in which said hydrocarboncut, a stream HYD of makeup hydrogen and optionally a stream REC ofrecycle hydrogen are mixed to form an overall feed which is supplied tothe inlet to at least one reactor comprising a desulphurizationcatalyst, under operating conditions that can transform the organicsulphur-containing compounds of the HC cut into H₂S, in which the sourceor sources of hydrogen forming the stream HYD is selected, andoptionally at least one hydrogen purification treatment is carried outon HYD, REC or a mixture thereof such that said overall feed comprisesat most 50 ppmv of CO and at least 120 ppmv of CO_(x), whereCO_(x)═CO+½CO₂.
 2. A process according to claim 1, in which the sourceor sources of hydrogen forming the stream HYD is selected and at leastone hydrogen purification treatment is carried out on at least a portionof HYD so that HYD comprises at most 50 ppmv of CO and at least 120 ppmvof CO_(x).
 3. A process according to claim 1, comprising at least onehydrogen purification treatment T1 carried out on HYD, REC or a mixturethereof, said treatment T1 comprising carrying out limited CO₂elimination to obtain at least 200 ppmv of CO₂ in the overall feed.
 4. Aprocess according to claim 1, comprising at least one treatment T2 forthe purification of hydrogen carried out on HYD, REC or a mixturethereof, said treatment T2 comprising carrying out catalytic oxidationof CO by O₂ and/or H₂O to obtain at most 50 ppmv of CO in the overallfeed.
 5. A process according to claim 1, comprising at least onetreatment T3 for the purification of hydrogen carried out on HYD, REC ora mixture thereof, said treatment T3 comprising carrying out catalyticmethanation of CO by H₂ to obtain at most 50 ppmv of CO in the overallfeed.
 6. A process according to claim 4, comprising a hydrogenpurification treatment carried out on HYD, and optionally on REC or amixture thereof, said treatment comprising a treatment T2 of carryingout catalytic oxidation of CO by O₂ followed directly, and with nosecondary elimination of CO₂, by desulphurization of the HC cut in thepresence of a stream of the purified hydrogen.
 7. A process according toclaim 5, comprising a hydrogen purification treatment carried out onHYD, and optionally on REC or a mixture thereof, said treatment T3comprising carrying out methanation of CO by H₂ followed directly, andwith no secondary elimination of CO₂, by desulphurization of the HC cutin the presence of a stream of the purified hydrogen.
 8. A processaccording to claim 6, comprising a conducting said treatment T1 foreliminating CO₂ on HYD upstream of T2 or T3.
 9. A process according toclaim 1, comprising a hydrogen recycle REC ard a purge of recycledhydrogen WGAS in which the flow rate of the purge WGAS is controlled sothat the CO content in the overall feed is less than 50 ppmv but thatthe COX content in the overall feed is more than 120 ppmv.
 10. A processaccording to claim 1, for hydrodesulphurization of an olefinic gasolinecut, carried out on a catalyst which comprises at least one element fromgroup VIII and optionally an element from group VIB, the element fromgroup VIII being selected from the group constituted by nickel, cobaltand iron, the optional group VIB element is molybdenum or tungsten, inwhich the reactor pressure is in the range 0.5 MPa to 5 MPa, the ratioof the flow rate of hydrogen in normal litres of hydrogen per hour tothe flow rate of hydrocarbons in litres per hour is in the range 50 to800, and the temperature is in the range 200° C. to 400° C.
 11. Aprocess according to claim 10, in which the catalyst comprises cobaltand molybdenum.
 12. A process according to claim 2, comprising at leastone treatment T2 for the purification of hydrogen carried out on HYD,REC or a mixture thereof, said treatment T2 comprising carrying outcatalytic oxidation of CO by O₂ and/or H₂O to obtain at most 50 ppmv ofCO in the overall feed.
 13. A process according to claim 3, comprisingat least one treatment T2 for the purification of hydrogen carried outon HYD, REC or a mixture thereof, said treatment T2 comprising carryingout catalytic oxidation of CO by O₂ and/or H₂O to obtain at most 50 ppmvof CO in the overall feed.
 14. A process according to claim 2,comprising at least one treatment T3 for the purification of hydrogencarried out on HYD, REC or a mixture thereof, said treatment T3comprising carrying out catalytic methanation of CO by H₂ to obtain atmost 50 ppmv of CO in the overall feed.
 15. A process according to claim3, comprising at least one treatment T3 for the purification of hydrogencarried out on HYD, REC or a mixture thereof, said treatment T3comprising carrying out catalytic methanation of CO by H₂ to obtain atmost 50 ppmv of CO in the overall feed.
 16. A process according to claim12, comprising a hydrogen purification treatment carried out on HYD, andoptionally on REC or a mixture thereof, said treatment comprising atreatment T2 of carrying out catalytic oxidation of CO by O₂ followeddirectly, and with no secondary elimination of CO₂, by desulphurizationof the HC cut in the presence of a stream of the purified hydrogen. 17.A process according to claim 13, comprising a hydrogen purificationtreatment carried out on HYD, and optionally on REC or a mixturethereof, said treatment comprising a treatment T2 of carrying outcatalytic oxidation of CO by O₂ followed directly, and with no secondaryelimination of CO₂, by desulphurization of the HC cut in the presence ofa stream of the purified hydrogen.
 18. A process according to claim 14,comprising a hydrogen purification treatment carried out on HYD, andoptionally on REC or a mixture thereof, said treatment T3 comprisingcarrying out methanation of CO by H₂ followed directly, and with nosecondary elimination of CO₂, by desulphurization of the HC cut in thepresence of a stream of the purified hydrogen.
 19. A process accordingto claim 15, comprising a hydrogen purification treatment carried out onHYD, and optionally on REC or a mixture thereof, said treatment T3comprising carrying out methanation of CO by H₂ followed directly, andwith no secondary elimination of CO₂, by desulphurization of the HC cutin the presence of a stream of the purified hydrogen.